Operation of electrical circuits is limited by interference. Thus, interference precludes optimal performance. Applications affected by interference are widely known and include signal conversion, amplification, measurement, and demodulation.
As is known, one approach to reducing effects of interference includes the use of one or more filters. These filters typically are electrical or electromechanical. However, filtering can only significantly diminish frequency components of interference outside the passband of a signal (or signals) of interest. Moreover, filters often distort the signal of interest within the passband. Nonetheless, filtering remains the accepted practice for reducing the effects of noise and other interference.
Another approach to reducing the effects of interference is averaging. In averaging, several sample values or records of a voltage are obtained. These several measurements are then averaged. Unfortunately, the benefits of averaging may not be realized in real time. Moreover, this provides only an average value of the signal of interest, which may not sufficiently accurately correspond to the actual signal of interest. Lastly, if the mean value of the interference is not zero and is unknown, then the resolved signal, the average value of the signal of interest, is biased by the non-zero mean value of the interference.
Another approach to reducing the effects of interference is correlation. Correlation is also an averaging process. Consequently, the degree of interference reduction depends on the averaging time allowable for the application. Cancellation of interference in real-time is not possible using correlation. As correlation requires a priori knowledge of the signal to be effective, correlation is primarily used in signal detection applications, e.g., determining the presence or absence of a signal of known structure.
Due to the limitations of filtering, averaging, and correlation, reducing in real-time the effects of interference in the passband of a signal is desirable.
The inventor of the present invention has U.S. Pat. Nos. 4,859,958 ("patent '958") and 4,992,747 ("patent '747"), which are both incorporated by reference as though fully set forth herein. In these patents, a means of demodulation of all of several FM carriers is described. U.S. Pat. No. 5,038,115 ("patent '115"), co-invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. In patent '115, phase tracking of input signals is described. In one embodiment of the phase tracking circuit of patent '115, a phase tracking circuit makes use of two phase-locked loops electrically connected in a feed-forward manner.